In a statement sent to the chairs of Congressional committees, Comey wrote:

“In connection to an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation. I am writing to inform you that the investigative team briefed me on this yesterday, and I agreed that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information, as well as to assess their importance to our investigation.”

NBC News's Pete Williams reports that the emails were found on “a device” during a separate investigation. The emails, his sources said, were not from Clinton. According to the New York Times, the emails were found on seized devices of Anthony Weiner and Huma Abedin in separate investigation into Weiner's sexting with underage girls.

The Associated Press reports that the new emails were not on her private server:

BREAKING: US official: Newly discovered emails related to Clinton investigation did not come from her private server.

It suggests something big led to Comey's decision to look into the case again.

In a statement following the FBI's announcement, House Speaker Paul Ryan said:

“I renew my call for the Director of National Intelligence to suspend all classified briefing for Secretary Clinton until this matter is fully resolved.”

When asked about the reopening of the probe, a top campaign spokesperson told NBC News they had “no idea.” Later in the afternoon, campaign chair John Podesta called on the FBI to release more information: